In the manufacture of tissue paper and paper towel a web of paper fibers is formed and pressed against a Yankee dryer. After drying the web is scraped from the surface of the Yankee dryer, giving the web a creped texture which gives the paper it's soft absorptive characteristics. Because only a single dryer is used the Yankee dryer is normally large, typically from twelve to twenty-five feet in diameter. Moreover, the Yankee dryer is heated by steam at pressures of up to 160 psig. A Yankee dryer may be 400 inches long and may have a total weight of over 100 tons. Because of its large size and high operating pressure, a Yankee dryer typically has a cylinder wall thickness of over two inches. Yankee dryers are generally formed from cast iron; a material which has good release characteristics if the surface is properly ground. Thus the surface of a Yankee dryer requires periodic regrinding to maintain the proper surface finish.
A Yankee dryer is a pressure vessel and the safety precautions typically employed with any pressure vessel or boiler must be observed. In U.S. Pat. No. 4,398,421 an apparatus for measuring the thickness of a work piece which is useful for measuring the thickness of steam boiler walls is disclosed. Determining the wall thickness of a Yankee dryer is useful but it is also desirable to detect small voids within the thickness of the dryer wall.
Existing ultrasonic inspection systems have a limited capability for detecting small voids or finding voids near the surface of cast iron parts. Ultrasonic signals in cast iron are scattered and reflected from grain boundaries present in the cast iron. This characteristic of cast iron makes detecting small discontinuities very difficult. Even measurement of thickness in cast iron can be difficult to perform. One national study has found errors of over 40 percent in thickness measurements of cast iron with some conventional techniques. Ultrasonic transducers typically have an interface zone of up to one-half inch where subsurface discontinuities can not be detected.
X-ray methods are used for the inspection of Yankee dryer rolls. X-ray methods, however, require the use of radioactive sources which are cumbersome and dangerous. In practice, x-ray images are only made of limited portions of the Yankee dryer. Furthermore, x-ray imaging is not effective at detecting hair-line cracks because such cracks do not significantly reduce the density of the material. Hair-line cracks are, however, typically detectable by ultrasonics.
In the application No. 08/690,763 filed Aug. 1, 1996, which is incorporated herein by reference, a method of ultrasonic inspection was disclosed which is capable of detecting flaws in a Yankee dryer with a high signal-to-noise ratio.
In that application it was disclosed that if an ultrasonic signal is directed at a surface to be inspected at a particular angle, about ten percent of the signal will appear as Harris waves which propagate into the test plate at ninety degrees to the surface. The signal which propagates vertically is polarized, with the result that the signal detects flaws with greatly improved signal-to-noise ratio. The particular angle is between that angle where the ultrasonic signal is refracted so as to propagate parallel to the surface of the metal plate and that angle where the ultrasonic signal is reflected by the test plate. Employing shear waves, for an incident medium of water and a refracting medium of cast iron, the specific angle is approximately 33 degrees from the vertical. For steel the specific angle is approximately 31 degrees. For brass the angle is about 50 degrees.
This particular angle while detecting flaws, produced a signal without an apparent time of flight--meaning that the depth of the flaw could not be determined with the disclosed method. Methods of finding the depth of a flaw using ultrasonic energy can be difficult to calibrate. Traditional methods require a flat bottomed hole drilled from a surface opposite the surface from which the ultrasonic interrogation is conducted. A hole with a flat bottom is difficult to drill and difficult to precisely line up parallel to the interrogation surface.
What is needed is a method for performing complete inspection of a Yankee dryer for material defects which can determine the depth of any detected flaws.